1. Field of the Invention
The present invention relates generally to interleaving data in tape drives. In particular, the present invention provides methods and apparatus for writing interleaving data to a multitrack tape, data-write methods and tape drives incorporating such interleaving systems.
2. Description of the Related Art
In multitrack tape drives, a plurality of write elements are provided on the write-head for writing data simultaneously in respective parallel data tracks extending longitudinally within the tape. In LTO (linear tape open) drives for example, multiple data tracks are arranged in several data bands spaced laterally across the ½-inch (1.27 cm) magnetic tape. The LTO standard is detailed in Standard ECMA-319, “Data interchange on 12.7 mm 384-track magnetic tape cartridges—Ultrium-1 format”, June 2001. For each data band on the tape, the read/write head can be positioned in a plurality of lateral positions based on the positional information derived from servo patterns recorded on the tape. The servo patterns are recorded in servo bands located on either side of a data band, and are read by dedicated servo sensors on the head. The TBS (timing-based servo) format is used in LTO drives. The TBS servo pattern has a frame format where magnetic transitions define a series of stripes with two different azimuthal slopes.
During read/write operations, the transversal position of the head can be derived from the relative timing of pulses generated by a narrow servo sensor that reads the stripe pattern. A plurality of servo locations are defined, where each servo corresponds to a different lateral position across the servo pattern, and hence resulting in a different relative pulse-timing. Each servo location corresponds to a different lateral position of the multi-element write head across the data band. In each of these lateral positions, the head can simultaneously write to a different subset of the tracks in the data band. TBS patterns also allow the encoding of longitudinal position information by shifting transitions (stripes) in the servo pattern from their nominal pattern position in the longitudinal direction of the tape.
In LTO drives, the read/write head can simultaneously write to 2n data tracks using 2n write elements as described above. The required reliability of data read from the tape is very high, e.g., on the order of one erroneous byte in 1017 read bytes. The LTO recording channel employs an error-correction (EC) coding technique based on a two-dimensional RS (Reed-Solomon) product code. Shortly after various preliminary processing stages, a stream of input data symbols is partitioned into successive “data sets”. Each data set is further partitioned into m*2n data blocks, known as “sub data sets”, where m and n are positive integers and * denotes multiplication herein. Each sub data set has a logical array of K2 rows and K1 columns of data bytes. EC coding is then performed over each row and column of the sub data set using a two-dimensional RS product code, where (N1−K1) check bytes are added at the end of each row and (N2−K2) check bytes are added at the end of each column. The resulting encoded sub data set has N2 rows and N1 columns of data bytes. The rows and the columns of the encoded sub data sets are referred to as C1 codewords and C2 codewords respectively.
After various further processing stages, the C1 codewords of the m*2n sub data sets are supplied to the 2n write elements on the read/write head that write simultaneously to 2n data tracks. The assignment of codewords to particular tracks, and the ordering of codewords in each track, is determined by an interleaving table defining a logical array of rows and columns of locations. This interleaving table has 2n columns, one for each of the 2n simultaneously-written data tracks. C1 codewords from the m*2n sub data sets are effectively assigned to respective locations in the interleaving table. The sequence of codewords in each column of the interleaving table is then supplied (after various additional processing stages) to a different one of the 2n write elements and written in the associated data track.
Examples of interleaving systems for use in LTO drives are described in co-pending U.S. patent application Ser. No. 12/351,738, filed Jan. 9, 2009, and Ser. No. 12/351,747, filed Jan. 9, 2009. These systems provide a uniform distribution of C1 codewords from the same sub data set in the interleaving table. U.S. Pat. No. 6,282,039B1 similarly discloses a system for uniform distribution of codewords from two-dimensionally coded data in a tape storage system.
The capacity and performance of tape storage systems using such multitrack drives have increased considerably in recent years, but there is still a need to improve efficiency. In existing techniques to improve tape-cartridge capacity, tape media have been made thinner and track widths and recorded bit-lengths have been reduced to increase data storage density. Our co-pending U.S. patent application Ser. No. 12/353,106, filed Jan. 13, 2009, describes a tape storage system in which the servo band width on tape is reduced and 2n+1 tracks are recorded simultaneously. However, while increased capacity is important, high read-back reliability is also paramount and thus an improved system would be desired.